This invention relates generally to a switching circuit and particularly to a switching circuit for rapidly breaking an electrical connection between a power amplifier and a loudspeaker upon cutting off an a-c power source.
Various amplifiers of direct-coupled type having no output condensor find extensive use in audio-frequency power amplifiers and other applications. In such audio-frequency power amplifiers, there is sometimes employed an a-c power source, from which a suitable d-c power is obtained through rectifying and filtering means and then fed to the amplifier. However, in the above case of making use of the a-c power source, some disadvantages are encountered as described below. When the a-c power supply is cut off, there is unwanted noise from a loudspeaker connected to the amplifier, which is generated by a direct voltage arising from an unbalance of direct current in the amplifier. Furthermore, when conditions are at the worst, there is a fear that the loudspeaker suffers irrecoverably from the excessive direct current flowing through its voice coil due to the above-mentioned unbalance. Therefore, there have heretofore developed several protecting circuits of the loudspeaker from such undesirable conditions. Each of the prior art for the purpose gain success in preventing the loudspeaker from being destroyed, whereas it is not sufficient to eliminate the noise from the loudspeaker, in that it can not respond quick to the cut-off of the a-c power source.